SubZero - Square's Bitcoin Cold Storage solution

Square stores a reserve of Bitcoins in an offline setting. By having these funds offline, it reduces attack surface and risk of theft. Square's solution is unique, specifically, it leverages FIPS certified Hardware Security Modules (HSMs) to protect the private key material. Funds can be sent from online systems to the cold storage at any time. Moving funds out of cold storage requires a multi-party signing ceremony.